Sedimentary Basins of the Republic of Yemen : Their Structural Evolution and Geological Characteristics

Abstract

The distribution and evolution of the sedimentary basins of Yemen was, until recently, poorly understood as this was based entirely on surface geology and correlations of the older stratigraphic units which were exposed only in the deeply dissected bordering uplifts of the Gulf of Aden and Red Sea or the high plateau of the north west. Elsewhere cover by the tabular Tertiary sedimentary blanket and the Tertiary Volcanic Group lavas masked the major underlying pre-Cenozoic structural elements and sedimentary successions. Earlier attempts at the delineation of the country's structural framework were, thus, sketchy and/or only partially correct. The discovery of commercial oil and gas in several interior Mesozoic rift basins of Yemen in the late 1980s and in the early 1990s after unification of the former two Yemens, spured many oil companies to enter the exploration race and carry out detailed seismic surveys and intensive exploration drilling in many areas. This resulted in a rapid rise in overall new subsurface geological data acquisition and an increasingly clearer perception of the distribution, orientation and inception times of the main basins. No overall synthesis of results was, however, undertaken since each individual company was primarily concerned with its own concession area and its immediate surroundings. Recent studies involving the review, correlation and synthesis of the mass of new subsurface stratigraphic data in connection with standardisation of lithostratigraphic nomenclature in use in Yemen and its further formalisation in accordance with internationally accepted rules, have, perforce, required the establishment of an overall structural framework within which inter and intra-basinal stratigraphic correlation could be carried out. It is this new framework of depositional basins and interbasinal uplifts that is discussed here. The main Mesozoic basins are related to late Jurassic extension and rifting, principally involving rejuvenation of the ancient NW-SE Najd fault system with subordinate strike-slip motion. The Tertiary basins are related to separation of Arabia from Africa in the Neogene along the Gulf of Aden and Red Sea trends. The principal uplifts are of equal antiquity, the oldest and most persistent being the Hadhramawt Arch. All the tectonic activity is linked to Gondwanan fragmentation and breakup from the initial closure of the paleo-Tethys and opening of the neo-Tethys in late Carboniferous to Permian times, and the development of the African Karoo system, through separation of India from Afro-Arabia in the Cretaceous to ultimate separation of Arabia from Africa in the Neogene. Within individual basins, compartmentalisation by sub-parrallel highs into graben-like and half-graben sub-basins or sectors is related to local adjustments to this continuing motion within the overall structural framework.